My take is that it has little or no effect on fat accumulation, and that lean people exercise because they’re lean; they don’t become lean by exercising. I do think it’s good for us, and it may even make us healthier in the short and the long run — although those assumptions could use some rigorous testing. I am a longtime jock myself and a devotee of no–pain–no–gain type gym workouts and hikes. But I just don’t think it makes a damn bit of difference when it comes to long–term fat accumulation. And this has been demonstrated time and again in clinical trials since the 1980s, and can be understood down to a molecular, enzymatic level, if anyone cares to look at the evidence. As I explain in my books, what exercise does is make you hungry. It doesn’t make you lean. Remember the concept of “working up an appetite?” Well, that’s what you do when you work out: You work up an appetite.
One caveat here, though. Every time I say publicly that I don’t think exercise has any meaningful effect on fat accumulation — and by exercise, in this context, I really mean increasing energy expenditure — my friends in the strength–training community reprimand me for what they think is compelling evidence that resistance training can induce fat loss, specifically in the context of low–carbohydrate diets. This may indeed be true, but I haven’t seen enough evidence personally to say it is.
And even though I don’t believe it’s a lack of resistance training that causes us to get fat in the first place, it’s possible, and I’d like to avoid the usual e–mails from my friends, so I want to mention it here.